The 2000 Burmister Lecture
Department of Civil Engineering and Engineering Mechanics
Impacts on Geotechnical Engineering of several recent
findings from laboratory stress-strain tests on geomaterials
University of Tokyo
October 31, 2000
2:30 - 3:30 p.m.
Davis Auditorium, Schapiro Hall
The lecture will consisted of the following four topics:
For the first topic, recent developments in the characterisation of geomaterial pre-failure deformation properties will be reviewed, focusing on the data required to predict ground deformations and structural displacements at working loads. Descriptions will be given of the deformation characteristics developed at very small to intermediate strains, of a variety of geomaterials, in testing using modern laboratory and field techniques. The relationships between static and dynamic experiments, between laboratory and field techniques, and between testing and field full-scale behaviour, which has been rather understood in a separated manner, will be discussed. Important features that will be highlighted include:
For the second topic, results from a series of triaxial compression and extension tests, plane strain compression tests and torsional shear tests performed on reconstituted specimens of different sands and gravels obtained from different countries and a undisturbed sand will be presented. It will be shown that commonly with these granular materials, the strength and pre-peak deformation properties are markedly anisotropic. It will also be shown that the strength and deformation properties obtained from different testing methods can be linked to each other only when taking into account the effects of inherent anisotropy, among other parameters.
For the third topic, results from a series of special plane strain compression tests performed on a number of different granular materials having a wide range of particle size and sedimentary soft rocks will be presented. It will be shown that how strains are localised as approaching the peak stress state, resulting into shear banding in the post-peak resume. The effects of particle size, in particular, on the thickness and the shear deformation and dilatancy characteristics of shear band will be demonstrated.
Related to the second and third topics, results from a series of model bearing capacity tests of footing on sand, which are unreinforced and reinforced, will be presented. It will be shown that how progressive the failure of ground is in the sense that the local peak strength is never mobilised simultaneously along the potential failure plane. It will be shown that the scale effects in the bearing capacity of footing on uncemented granular materials are due to not only the effects of confining pressure on the peak friction angle (i.e., the pressure level effects), but also the changes in the particles size to the footing size (i.e., the particle size effects). It will be shown that the failure of a mass of dense granular material can be reasonably numerically simulated (by the FEM) only when the inherent anisotropy in the strength and deformation characteristics and the deformation characteristics of shear band (as a function of particle size), among other parameters, are taken into account. It will be shown how careful engineers should be when using classical bearing capacity theories in engineering practice.
Finally, for the last topic, the prediction of long-term residual deformation of ground and displacement of completed structures under sustained load will be discussed. Based on several case histories of field full-scale behaviour of backfill, ground and proto-type structures, two important factors; a) effects of recent strain history on the creep rate; and b) time-dependent stress-strain behaviour under unloaded conditions, including the creep recovery phenomenon, will be focused. It will be shown how erratic results are obtained from conventional creep tests on stiff geomaterials, but local axial strain measurement is a must for such tests. Results from laboratory stress-strain-time tests and model loading tests that show the above will be presented. Several basic theoretical issues related to constitutive-modelling of the stress-strain-time property of geomaterials will also be discussed. The isotach property will be discussed and a new model will be proposed.
Note: a handout for the lecture will be distributed. Interested
readers may request for a mailed copy.